With a mechanical seal, which is an example of a sliding component, the mutually exclusive conditions of “seal” and “lubricate” must be satisfied simultaneously in order to maintain its sealing performance for an extended time. Particularly in recent years, to help protect the environment, etc., there is a growing need to lower friction further and thereby reduce mechanical loss, while still preventing leakage of the fluid being sealed. Friction can be lowered by creating a so-called fluid lubrication state, which represents a state of surfaces sliding against each other with a liquid film in between, achieved by generating dynamic pressure between the sealing faces by means of rotation. In this case, however, generation of positive pressure between the sealing faces causes the fluid to flow out of the sealing faces from the positive pressure area. This is so-called side leakage that occurs with a bearing, which corresponds to leakage in the case of a seal. When the fluid to be sealed is present on the outer periphery side of the seal area and atmosphere on the inner periphery side, and the fluid on the outer periphery side is sealed in this state (referred to as the “inside type”), the leakage rate on the inner periphery side is expressed by the formula below:
                    Q        =                  -                      ∫                                          (                                                                                                    h                        3                                                                    12                        ⁢                                                                                                  ⁢                        η                                                              ⁢                                                                  ∂                        p                                                                    ∂                        r                                                                              ⁢                                      |                                          r                      =                                              r                        1                                                                                            )                            ⁢                                                r                  1                                ·                                  ⅆ                  θ                                                                                        {                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          1                }            
Q: Leakage rate on the inner periphery side at the inner diameter r1 of the sealing face (The negative sign indicates leakage).
h: Height of clearance
η: Viscosity of fluid
p: Pressure
From the above formula, it is clear that the pressure slope ∂p/∂r at the inner periphery side increases as fluid lubrication is promoted, dynamic pressure generates, and liquid film forms, and as a result of a larger h, the leakage rate Q increases.
In the case of a seal, therefore, the clearance h and pressure slope ∂p/∂r must be decreased in order to reduce the leakage rate Q.
As for the friction characteristics of a slide bearing, which are similar to those of a mechanical seal, the “Stribeck curve” shown in FIG. 4 is known (Reference Literature: “Tribology” by H. Czichos, Kodansha).
The horizontal axis in FIG. 4 represents “Viscosity η×Velocity v/Load FN,” or simply the velocity if the viscosity and load are constant. If the viscosity and load are constant, the friction coefficient is small in the medium-speed region or mixed lubrication region “Second: h (clearance)≅R (roughness)” and the high-speed region or fluid lubrication region “First: h (clearance)>>R (roughness),” but the friction coefficient becomes extremely large at startup in the boundary lubrication region “Third: h (clearance) →0.”
According to the numerical analysis conducted by the inventors named in the present application for patent, on the other hand, the groove depth on the sealing face and friction coefficient of the sealing face have the relationship shown in FIG. 5 in the case of a mechanical seal, and the relationship between the groove depth and friction coefficient of the sealing face varies depending on the sliding speed of the sealing face.
Also, generally dynamic pressure-generating grooves are provided on a mechanical seal to ensure the mechanical seal is effective in the normal rotational speed region and also to guide sufficient fluid to the sealing face, and these dynamic pressure-generating grooves are processed to a depth of several μm or more by means of machining, blasting, or laser. Because of this, low friction is achieved in the medium-speed region and high-speed region, but sufficient load capacity cannot be achieved in the low-speed region, which makes it difficult to achieve low friction in this speed region. Particularly at startup and stop where sufficient dynamic pressure does not generate, sufficient lubrication characteristics are not demonstrated and problems occur as a result, such as noise and excessive contact between the sealing faces at startup and stop.
Also in recent years, sliding materials for mechanical seals are proposed that can reduce the friction coefficient without generating excessive leakage by introducing the sealed fluid to the space between the sealing faces and holding it there in good condition, including one where multiple dynamic pressure-generating grooves are provided in the circumferential direction to generate dynamic pressure between the sealing faces as a result of one sealing face rotating relative to the mating sealing face, wherein the dynamic pressure-generating grooves comprise straight grooves or curved spiral grooves having an angle to the sliding direction and the dynamic pressure-generating grooves are processed to a depth of 1 μm or less by means of femtosecond laser (refer to Patent Literatures 1 and 2, for example).
However, the inventions described in Patent Literatures 1 and 2 aim to generate dynamic pressure between the sealing faces as a result of one sealing face rotating relative to the mating sealing face, and although low friction is achieved in the medium- and high-speed regions at high pressure, sufficient dynamic pressure is not generated and thus sufficient lubrication characteristics cannot be demonstrated in the medium- and high-speed regions at low pressure or at startup and stop, which presents a problem. In addition, the mechanism of introducing the sealed fluid into the space between the sealing faces by the dynamic pressure-generating grooves requires circular grooves or other means for preventing leakage to be provided on the low-pressure side of the sealing face, in order to reduce the leakage rate.